Preoxidation of stainless steel



April 4, 1950 W. E. KINGSTON PREOXIDATION OF STAINLESS STEEL Filed Oct. 18, 1944 WALTEK fikm's 70 V INVENTOR ATTORN EY Patented Apr. 4, 1950 PREOXIDATIONOF STAINLESS ,STEEL Walter E. Kingston, Port Washington, N. Y.,.v assignor to Sylvania Electric Products Inc.,f;Emporium, Pa, a corporation of Massachusetts-- 1 Application October 18, 1944 Serial No. 559,227

1 Claim. (Cl. 148-6.35)

This invention relates to stainless steel alloys, such for example as those used in metal-to-glass seals and more particularly to a process of preoxidation of the surface of stainless steel parts prior to the step of sealing the glass to the metal surface.

Certain chromium nickel iron alloys adapt themselves particularly well for matching their thermal expansion coeihcient to that of a "number of glasses over a wide temperature range, whereby internal stresses in a composite body made by sealing the alloy surface to a properly matched lass are reduced to a minimum. In order to obtain a uniform, vacuumtight and mechanically strong bond between the metal surface and the glass to which the metal body is to be sealed, it is necessary to provide a uniform layer of metal oxide on the surface of the metal body, which oxide is soluble in the glass and which adheres very strongly to the metal itself. While an oxide layer of this character can be obtained on the surface of chromium bearing alloy parts by exposing them at elevated temperature to an atmosphere of wet hydrogen, great diificulty has, however, been experienced in obtaining a uniform, thin chromium oxide coating. This results from the fact that variations in the thickness and in the composition of the surface oxide may easily occur if the processing is not :properly controlled.

In order to obtain the proper kind of surface oxide, it is necessary that the external surface of the alloy be covered with a self-protective chromium oxide layer of suflicient density and thickness. This dense protective layer prevents the penetration of oxygen below and into the lower levels of the alloy body during the sealing in process. Should a considerable amount of oxygen penetrate during the sealing-in, the result would be the formation of a ferrous oxide, which is flaky in character and is the cause of air leaks.

In the mass production of electron discharge tubes, it is essential that all metal-to-glass seals be perfectly vacuumtight when subjected to standard high speed sealing methods. As an example, in the production of glass stems or headers for electron discharge devices, through which lead-in conductors of chromium-nickeliron alloys are to be sealed, the lead-in conductors must have a well defined kind of oxide on their surface. It is important that this surface layer must not contain the ferrous oxide, which flakes off the metal surface and results in air leaks because of the separation between the metal surface and the oxide layer in the finshed seal.- In view of the fact that the lead-in conductors 2 a are frequently used as contact prongs to be introduced directlyinto the tube socket, it is neces sary that the oxide formed during the pre-oxida-' tion can be easily removed from the external parts of the lead-in conductors, so as to insure a good metallic electrical conductive contact en: gagement between the lead-in conductors of the tube and the socket contacts. Furthermore, the surface oxide must be removed fromithe parts of the lead-in conductors inside the discharge tube; to which are to be welded the tube electrodes-as the mechanical and electrical properties of the connection between the tube elements and lead-in conductors depend to a large extent on the absence of oxide on the surfaces to be welded together.

I have found that the uniformity of the desired oxide layer, in particular of lead-in conductor pins made of a 'chromium-nickel-iron alloy can be obtained with maximum speed by following a heat treatment of the pins which consists of a number of steps which form the subject matter of my present invention.

It is, therefore, an object of the inventionto provide a method of pre-treating the surface of chromium-nickel-steel bodies to be sealed to glass in a vacuum-tight manner, so as to insure a uniform and stable oxide layer of the kind desired for this purpose. It is another object of the invention to provide a reliable method of pre-oxidi'zing chromium-1" nickel-steel parts in a controlled atmosphere; whereby a thin; uniform chromium oxide layer; tightly adherent to' the alloy surface is produced in the shortest'possible periodoftime; It is still a further object of the inventionto provide" a method of pre-oxidizing'chrom'iumnickel-steel parts to'be'sealed to" glassjin' which the formation of afiaky'iron oxide is-completely I eliminated." T

to a stream of hydrogenat low temperature prior to their exposure to the controlled atmosphere at high temperature.

A feature of the invention relates to the tentperature-timeschedule to 'be applied for pre: oxidizingchromium-nickehsteel parts in a conz-F trolled atmosphere, for the purpose of obtaining a vacuum-tight metal-to-glass seal on apart ofthe metal parts, which oxide can be easily re-' moved 'from the surface of the parts where a good electrical contact is desired.

The invention will now be described in connection with the drawing, in which,

Fig. 1 shows a bottom view of a finished glass header for an electron discharge tube, including chromium nickel alloy pins sealed through the header after .pre-oxidation according to the invention.

Fig. 2 is a sectional view ofFig. 1, along line 22.

Fig. 3 is a schematic sketch of a furnace illustrating my new method of oxidation.

Referring now to Figs. 1 and 2, the contact pins 3 sealed through beaded or thickened portions 5 of a glass header l, which may :be formed in the shape of a cup, with a skirt portion 4, may be made of an alloy consisting of 38'to 45% nickel; 3 to chromium; up to 2% oi. Ta metal which forms a comparatively refractory and stable oxide when formed at high temperatures, such as a metal of the group consisting of aluminum, zirconium and .calcium; the ;rest of :the alloy being substantially entirely viron. This alloy matches particularly well the expansion coefficient of a glass consisting of 63% S102; PhD; 0.28% A1203; 10.94% CaO; 7:6'% .NazO; 5:5% K203 .and 0.88% .MnzOs. A preferred composition .of the .alloy for making the lead-fin conductor is approximately 42% Ni; 5 to 18% Cr; .02 to 2% .Al; balance substantially iron. The contact pins 3 haveadiameter large enoughto make them sumciently strongand-rigid, so their lower parts may bepused'asscontact prongs for insertion .into a'tuhesocket, .and their upper ends can serve as main supports for discharge electrodes to be 'joined to-the vpins .by welding.

The part of each pin sealed into the .glass header is'to be'pre-oxidizedinithe manner about to :be described, for the purpose of forming a yacuumtight, mechanically strong metal-to-glass seal. As explained above, the upper and lower ends of thepins should'be freed from oXide after the metal-to-glass seal has been made.

In order to obtain pins which have theproper oxide for making the :metal-to-glass seal, and

fromwhichthe oxide can be removed at the upper andlower portions extendingabove andbelow the glass header, the pins are loaded vin-metal .trays and passed through-a controlled atmosphere at high temperature. The atmosphere consists of hydrogen whichis saturated with water vapor at aftempera'ture of 80ito 100'F. In the mass production of stems of this type, itiis desirable that theprocesscof pro-oxidizing be carried out at high speed, and high temperatures ".of the controlled wet hydrogen atmosphere are, therefore, 'de-. sirable.

.Iti'has been found that the desired thin, .uniform oxide layer-cannotbe obtained at temperatures above 1600 unless special precautions are taken. At this temperature, the desired oxide W111'2'0ll1y form after an exposure of several hours. If the temperature of the oxidizing at mosphereis elevated,.-say to 2300 F., in order to accelerate the formation of the oxide, the surfaceof the pins will not oxidize uniformly, but a heavy oxide develops in some parts of the surface, and flaky iron oxide is formed in others. This phenomenon results from .the fact that the air introduced with the -pins ;into thehot chamber :causes irregularly I distributed high oxygen concentratlons'at the surface of the pins, which, at the sudden exposure to the high temperature in ithe =furnace disturb the critical composition of the :controlled atmosphere, and causes the formation of undesired iron ioxidein spots,

4 and leads to a varying thickness of chromium oxide over the surface.

Accordin to the invention, this effect is eliminated by flushing the air from the metal parts prior to introducing them into the high temperature chamber. This insures the uniformity of the composition of the controlled atmosphere when the metal parts are introduced.

If the pins are introduced into a stream of hydrogen for about one minute at a temperature of less than 200 F., all the adherent air is flushed .out, and the pins obtain their desired uniform thin oxide surface within about 30 minutes exposure to the wet hydrogen atmosphere,

at a temperature .of about 2300 F.

In practiceythe pre-oxidizing is carried out in a furnace illustrated in Fig. 3. A tube (0 is supplied with a flow of wet hydrogen at a point II. The tube is divided into three temperature zones. An entrance zone or chamber l2 of tube 10 is kept at about F.; the main oxidizing zone I3 is kept at about 2300 F.; and cooling zone '54 :is :kept at a temperature between about 200.and1005F. The ends of the tube are provided with hinged doors indicated as IE5 and 16 in Fig. 3. When the oxidizing process is started, door I5 is opened, and the hydrogen flow is increased to about three times its normal flow. A metal tray [1 containing the pins is introduced into entrance chamber |2 into position 1, and door l5 is closed again. Door l5 has two openings, a small opening l8 through which hydrogen passes outacontinuously. The escaping hydrogen is ignited, and forms the flame l9, whose length indicates the hydrogen pressure. A second larger opening .20 is provided in door l5 and high rate of hydrogen flow is kept up for about one minute to insure a speedy removal of the air adherentto the pins in the tray I! by the hydrogen -stream. Opening .20 can be opened and closed by a hinged shutter 2|, and is kept fully open during the flushing period. After this periodof time, tray I! can be safely introduced into heating chamber l3 at the high temperature, where it remains for about 30 minutes in position 2. The chamber 13 is heated to the desired temperature by any suitable electric or gas heater 22. The flow of hydrogen is reduced during this period and opening 20 of door 15 is closed, leaving only the small opening l8 for the hydrogen to escape.

After this heating for 30 minutes, the tray I! is successively pushed into positions 3, 4 and 5 in cooling chamber l4. Preferably the cooling chamber I4 is made long enough to permit further graduated cooling zones 23, 24, 25, before the tray is removed through door 56 on the exit end of the furnace. An opening 25 is provided in door It, as usual, to permit a continuous flow of hydrogen through this end of the furnace. If desired, a special cooling unit such as a water jacket 30 may surround the zone M, and if found necessary a similar water jacket may be provided around the zone :2.

The process may then be carried on continuously. :When the first tray I1 is moved from position 2 to position 3, another tray is placed into position 1, the hydrogen flow is increased for one .minute, and the new tray is moved into position ,2. After rthirty minutes, the new tray is moved into position ,3, and the first one into position 4, and a third tray is introduced into position 1. When the next tray is introduced into position 1, the first tray is pushed to posir tion #dand is ready forremoval to air. In this Way, a full tray of pre-oxidized pins may be taken from the furnace every 30 minutes.

The advantage of the new method of preoxidizing consists of the speed obtainable, and in the uniformity of the desired oxide layer. It will be noticed that the temperature in the oxidizing chamber I3 is about 400 F. below the melting point of iron. While it is possible to increase the oxidizing temperature slightly further in order to obtain a still higher speed of oxidation, this should be avoided, as at higher temperatures there is an inclination to develop a very loose heavy chromium oxide which is undesirable from a sealing standpoint. Also exaggerated grain growth results, which is detrimental to the physical and vacuum sealing properties of the alloy. It should also be noted that the hydrogen tank 21 is provided with a fiow regulator valve 28 and is bubbled through a water pan 29 so that it is saturated with water vapor when it enters the furnace.

What I claim is:

The method of pre-oxidizing the surface of chrome-nickel steel parts as a preliminary step prior to sealing them into glass, characterized in that the parts are exposed for about one minute to a high velocity stream of wet hydrogen at a temperature below 200 F. for the purpose of flushing away entrained air at the surface of the steel parts, and then in a wet hydrogen atmosphere at a temperature about 400 F. below the melting point of iron for about 30 minutes, and then are cooled for approximately 30 minutes in a hydrogen atmosphere at a temperature of less than 100 F. the hydrogen atmosphere having been previously saturated with water vapor at a temperature between 80 F. and 100 F.

WALTER E. KINGSTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

